Started From the Egg, Now We’re Here: Examining the Development of Consistent Individual Variation in Growth and Behavior of Captive-Reared, Precocial Wood Ducks (Aix sponsa)

Abstract

Individuals within a population often vary in the range and kind of behaviors that they exhibit. This variation, when consistent across time and/or context, has been termed animal personality. How and why consistent individual differences are maintained within populations when they have far-reaching fitness consequences remains a fundamental inquiry. Links between individual state and behavior have been hypothesized as an important component in the maintenance of this variation. Furthermore, understanding the development of consistent differences in state, behavior, and their relationship is of pivotal importance in addressing this question. With this dissertation we aimed to longitudinally examine the relationship between growth and personality across ontogeny in captive-reared Wood Ducks (Aix sponsa). In 2016 and 2017, we collected Wood Duck eggs from wild populations in the Central Valley of California and artificially incubated them at the University of California, Davis. We then reared hatched individuals in a controlled environment, collecting morphometric and behavioral data on a weekly basis using standardized assays. We found that individual differences in behaviors associated with sensitivity to predation risk (tonic immobility as well as activity, space-use, and response to a startling stimulus in a modified Open Field Test) remained relatively consistent across development. Repeatability of these behaviors qualitatively increased as ducklings aged while within-individual variance in behavioral expression decreased significantly, suggesting canalization. Duckling size also consistently varied and was significantly related to behavioral expression among-individuals, though the relationship was complex and age dependent. In a common garden setting, we found no evidence of feedback between size and behavior within-individuals as a driver of behavioral differentiation, suggesting that the observed variation in behaviors is not proximately driven by covariance with size during development. Experimentally increasing the perception of predation risk within the developmental environment significantly decreased size and growth rates as ducklings aged but had a limited effect on expression of select behaviors and their relationship with duckling size throughout ontogeny. Experimental treatments also appeared to generate negative feedback between a metric for Activity & Space-use and body mass within-individuals, potentially leading to lowered repeatability and convergence of this behavioral type. Herein we discuss the methodology of measuring consistent individual differences in behavior, the possible causes of the relationships we discovered, the implications of our findings for Wood Ducks (a precocial waterfowl species), and the broader implications of our study for the understanding of personality, its development, and its ecological impacts.